Dissipationless collapse and the dynamical mass-ellipticity relation of elliptical galaxies in Newtonian gravity and MOND

Abstract

Context. Deur (2014) and Winters et al. (2023) proposed an empirical relation between the dark to total mass ratio and ellipticity in elliptical galaxies from their observed total dynamical mass-to-light ratio data M/L = (14.1 +/- 5.4){\epsilon}. In other words, the larger is the content of dark matter in the galaxy, the more the stellar component would be flattened. Such observational claim, if true, appears to be in stark contrast with the common intuition of the formation of galaxies inside dark halos with reasonably spherical symmetry. Aims. Comparing the processes of dissipationless galaxy formation in different theories of gravity, and emergence of the galaxy scaling relations therein is an important frame where, in principle one could discriminate them. Methods. By means of collisionless N-body simulations in modified Newtonian dynamics (MOND) and Newtonian gravity with and without active dark matter halos, with both spherical and clumpy initial structure, I study the trends of intrinsic and projected ellipticities, S\'ersic index and anisotropy with the total dynamical to stellar mass ratio. Results. It is shown that, the end products of both cold spherical collapses and mergers of smaller clumps depart more and more from the spherical symmetry for increasing values of the total dynamical mass to stellar mass, at least in a range of halo masses. The equivalent Newtonian systems of the end products of MOND collapses show a similar behaviour. The M/L relation obtained from the numerical experiments in both gravities is however rather different from that reported by Deur and coauthors.